Radio Frequency (RF) power amplifiers are used in a variety of communication and other applications. In practice, power amplifiers behave in a non-linear fashion, which tends to distort the power amplifier's output signal. This distortion can cause several problems. For example, the distortion may result in transmission of signals outside of the desired transmitter channel. In some applications, government regulations set limits to these spurious emissions. Thus, spurious emissions are undesirable. In addition, these spurious emissions represent wasted power that would be better used in transmitting the desired output signal in the desired channel. Still further, these distortions can affect the demodulation of the transmitted signal when received by the intended recipient of the transmission. Accordingly, power amplifier distortion is undesirable.
Methods for reducing this distortion, commonly referred to as "linearizing" the power amplifier include "feedforward" linearization methods. FIG. 1 is a simplified functional block diagram of a conventional feedforward system 100. Feedforward system 100 includes a power amplifier (PA) 101, which receives an analog input signal to be amplified. A delay circuit 103 receives a signal that, ideally, is identical to the analog input signal received by PA 101, which serves to delay the "identical" analog input signal to match as closely as possible the delay of PA 101. A splitter or coupler (not shown) are commonly used to provide this "identical" signal to delay circuit 103. The output signal of delay circuit 103 is then received by a summer or combiner 105. In addition, combiner 105 receives, through a splitter or coupler (not shown), a signal that is ideally an attenuated but identical version of the output signal of PA 101. Combiner 105 then subtracts the "identical" analog input signal from the attenuated
output signal to generate an error signal. This error signal represents an attenuated version of the distortion in the output signal from PA 101.
The error signal is then received by a gain and phase adjuster (G/P) 107, which, ideally, amplifies and adjusts the phase of the error signal so as to match the distortion in the PA output signal. A combiner 109 is used to subtract the output signal from G/P 107 from the PA output signal, which is received from a second delay circuit 111 connected to receive the output signal of PA 101. Delay circuit 111 serves to delay the PA output signal so as to match as closely as possible the delay of G/P 107. Combiner 109, ideally, cancels the distortion in the PA output signal by subtracting the adjusted error signal from the PA output signal. The resulting signal is then broadcast through an antenna 120.
However, there are several shortcomings to conventional feedforward linearizers. For example, some feedforward linearizers are implemented using analog devices that are preset to provide a predetermined gain and phase adjustment to the error signal. Unfortunately, the distortion caused by the power amplifier generally changes over time due to temperature, age, power supply variations, etc., which cannot be compensated for by these "fixed" systems.
Other feedforward systems attempt to monitor the PA output signal and "adapt" the gain and phase adjustment of the feedforward error signal so as to minimize the error signal for a subsequent PA input signal. These conventional feedforward systems typically base the gain and phase adjustment of the error signal as a function of the instantaneous power or magnitude of the input signal to the power amplifier. Further, these types of systems tend to be systems that attempt to adjust G/P 107 using analog gain stages and phase shifters. These analog devices tend to be inaccurate and can "drift" over time.
Some adaptive feedforward systems use analog band-pass cross-correlation techniques to minimize the error signals in the system. However, analog band-pass cross-correlation techniques are sensitive to DC offset during the correlation process, resulting in poor linearization. In addition, analog band-pass cross-correlation techniques tend to have "masking" effects when correlating large signals with small signals.
To avoid problems associated with analog band-pass cross-correlation techniques, some adaptive feedforward systems use standard least mean squares minimization algorithms to minimize error signals in the system. These systems tend to require a large amount of data to get good results (i.e., slow convergence).
In view of the above shortcomings of conventional feedforward linearizers, there is a need for an adaptive feedforward linearizer that will compensate for power amplifier distortion more quickly and accurately than the conventional feedforward linearizers that are based on "real time" minimization and analog band-pass correlation techniques.